The oil and gas products that are contained, for example, in sandstone earth formations, occupy pore spaces in the rock. The pore spaces are more or less interconnected to define permeability, which is a measure of the ability of the rock to transmit fluid flow. If permeability is low, or when some damage has been done to the formation material immediately surrounding the bore hole during the drilling process, a hydraulic fracturing operation can be performed to increase the production from the well.
Hydraulic fracturing is a process where a fluid under high pressure is applied against the formation to split the rock and create fractures that penetrate deeply into the formation. The fractures provide additional flow channels, as well as more surface area through which formation fluids can flow into the well bore. The result is to improve the near term productivity of the well, as well as its ultimate productivity, by providing flow channels that extend farther into the formation. Most wells of this type are fractured upon initial completion, and are refractured at a later date to restore productivity To prevent healing of the fractures after the parting pressure is released, it has become conventional practice to use propping agents of various kinds to hold the cracks open, and spacer materials to ensure optimum distribution of the proppants.
During fracturing, fluids are injected into the formation at a given rate in order to initiate the fractures and then propagate them. Calibrations can be made to determine key design parameters, or propping agent treatments. The efficiency of fracturing treatments rely heavily on the ability to produce fractures that have optimum physical characteristics such as length, height, width and flow capacity. Such characteristics can be predetermined to some extent by using a reservoir model, together with certain selected economic criteria. A determination of the closure pressure, and the identification of fracturing events such as height growth and/or the occurrence of screenout (proppant bridging that restricts fracture extension), in a timely manner, is crucial to the economic success of a fracturing operation, and to any future operations in the same geographical area by appropriate modification of the design criteria.
It is known that fracture behavior and certain fracturing events cause characteristic changes or patterns of change, in downhole pressures. As an aid to pressure change pattern recognition from which a model that defines the fracturing process can be inferred, it is known in the art to plot net pressure values versus pumping time on a log-log scale, where net pressure is the difference between bottom hole pressure and the in-situ stress or fracture closure pressure. See Nolte and Smith U.S. Pat. No. 4,393,933 issued Jul. 19, 1983, and "Interpretation of Fracturing Pressures", Nolte and Smith, Journal of Petroleum Technology Sep. 1981, p. 1767. A low, positive slope for this net pressure plot indicates so-called "PKN" behavior where the fracture is one that penetrates deeply into the formation with height confinement. A low, negative slope of the plot indicates "KGD" behavior where fracture height is much larger than its penetration into the formation, and can also indicate a radial or a penny-shaped fracture. A portion of the plot that has a substantially flat slope is indicative of the opening of natural fissures in the rock and accelerated fluid leakoff. This phenomenon may result in "screenout", which, as mentioned above, is a condition where propping agents bridge the fracture and restrict further extension thereof. Screenout itself is characterized by a section of the plot that has a relatively high positive slope of about one, or even higher. The net pressure plot has served as a very useful pattern recognition tool for interpreting fracturing pressure data, and enables a diagnosis to be made of certain fracturing events.
However, the use of the net pressure plot depends upon the existence of certain input data which can be ill-defined. The time origin is when the fracture is initiated, which usually is taken to be the time at which the gelled fluids hit the formation. The slopes exhibited by the net pressure plot depend to some extent on the value of the closure pressure, which has to be measured independently, preferably using in-situ stress tests. Failure to have the actual closure pressure can result in an inaccurate slope of the plot. A net pressure plot with a small positive slope may appear to be flat if the closure pressure that was selected is too low, and vice versa. Consequently an inaccurate interpretation of fracture behavior can be made if the error is not detected. In addition, certain important fracturing events can be difficult to detect in a timely manner due to compression of the data that is imposed by a logarithmic scale. Thus, there remains the need to enhance pattern recognition techniques in a manner that will obviate the foregoing limitations, and enhance the sensitivity of the analysis.
The general object of the present invention is to provide a new and improved method of analyzing the pressure data during a well fracturing operation that enhances early identification of certain fracturing events, such as extension of a fracture with confined height, or with height growth, as well as early detection of the onset of screenout.
Another object of the present invention is to provide a new and improved method of analyzing pressure data during a well fracturing operation that enable a more accurate determination of minimum in-situ stress or closure pressure.